That's a good idea to welcome Halloween
I am by no means an expert in horror movies, but here's my little list.
In modern movies, I would suggest The Ring, definitely one of the best horror movies I've ever seen. The plot is so well built and the special effects are striking.
The Exorcism of Emily Rose would be another good pick. This is a movie I can't watch without a shiver
Not as good, but I also liked the Unborn.
Among the classics, the Nightmare of Elm Street are my favorites. I was way too young when I saw the first one and I remained somewhat traumatized by this film
Since then, I've seen the others several times and I still feel a delicious mix of amusement and apprehension while watching them.
I would also suggest Bram Stoker's Dracula from Francis Ford Coppola. The film is very well conceived, with its glamorous and gothic ambience and its impressive cast (Anthony Hopkins, Gary Oldman, Winona Ryder and more). The movie takes several liberties with the novel, but is equally enjoyable in its own way.
While at it, why not adding Alien, by Ridley Scott. Even if it's not a horror movie per se, the movie is fully horrifying, with its legendary monster and it's claustrophobic atmosphere.
I would end my list with the infamous Poltergeist from Steven Spielberg. The movie has aged and is no longer really horrifying, but it remains one of the best fantastic movies ever made. Nowadays, I think that the events surrounding the movie are scarier than the movie itself.
"From ghoulies and ghosties and long-leggedy beasties, And things that go bump in the night, Good Lord, deliver us!"